Read this excerpt from The Early History of the Airplane.

The larger works gave us a good understanding of the nature of the flying problem, and the difficulties in past attempts to solve it, while Mouillard and Lilientha, the great missionaries of the flying cause, infected us with their own unquenchable enthusiasm, and transformed idle curiosity into the active zeal of workers.

Using context clues, what does the term "zeal" most likely mean?

knowledge
confusion
enthusiasm
anger